Device for generating single photons and entangled photon pairs, method for generating entangled photon pairs, method for generating single photons

Abstract

An embodiment of the present invention provides a device for generating photons (300) comprising a quantum box (301) inserted into a micropillar-type optical cavity (302) having at least one optical mode, the quantum box (301) having at least one ground state and two states with one elementary excitation, the optical cavity (302) having a bottom surface and a top surface, the bottom surface having an electrical contact (305), and the photon generating device (300) advantageously comprising at least three electrical bonding pads (304a, 304b, 304c) electrically isolated from one another and arranged around the periphery of the top surface of the cavity (302).

Description

[Technical Field]

[0001] The present invention relates generally to sources of photons, and more particularly to devices for generating single photons and entangled photon pairs, and methods for implementing the devices. [Background technology]

[0002] The deployment of a quantum communication network requires the establishment of a single-photon source capable of generating a train of optical pulses, each containing one (and only one) photon. Such a source can provide two remote contacts with a secret key that allows subsequent communications to be encrypted. Imperfections in the propagation channel, whether optical fiber or free space, limit the range of such applications to several hundred kilometers. The range of a quantum communication network can be extended by quantum repeaters that rely on the entanglement of pairs of photons. The generation of pulses containing single photons or pairs of entangled photons can be performed by nanometer-sized emitters, such that the transition between their electronic states corresponds to the emission of a single quantum dipole.

[0003] Furthermore, the creation of quantum computers relies on systems that can be manipulated as qubits. A qubit is a coherent superposition of two fundamental states, usually representing |0> and |1>, that can adopt an infinite number of possible values, contrary to binary bits manipulated by conventional computers with values ​​of 0 or 1. Qubits can be either solid-state or photonic. The former type offers flexibility in terms of storage and processing, while the latter, photonic qubits, allow quantum information to be transported over long distances. It is common practice to use a combination of solid-state qubits and photonic qubits associated with an interface between the two elements to enable the processing and transport of quantum information.

[0004] The use of quantum boxes in applications including the deployment of quantum communication networks and the creation of quantum computers has experienced significant growth in recent years. The interest in such nanometer structures is explained by their similarity to atoms in terms of the confinement of charge carriers and the quantization of their energy levels. Such properties enable quantum boxes to generate single photons through the process of spontaneous emission in response to favorable light adaptation, which allows an electron to move from the valence band to the conduction band. The same mechanism can be adapted to place two electrons in the same energy level in the conduction band and generate entangled photon pairs. Quantum boxes also enable the reliable interaction of light with the properties of the confined charge carriers.

[0005] Isolated quantum boxes used as sources of single photons or entangled photon pairs are generally limited in terms of brightness and bit rate. While brightness quantifies the probability of having a single photon per optical pulse or an entangled photon pair per optical pulse, bit rate measures the number of photons emitted per second and is equal to the brightness, as defined above, multiplied by the rate of the radiation clock governing the optical pulses exciting the quantum box. To improve the performance level of quantum boxes in terms of brightness and bit rate, it is known to insert quantum boxes into resonant optical cavities. Such a configuration exploits the Purcell effect (also known by the expression "low light-material coupling system") to increase the spontaneous initial rate of quantum boxes in the cavity modes and enable more efficient collection of photons emitted by the quantum boxes. The encapsulation of the quantum box in an optical cavity also makes it possible to reduce the sensitivity of the quantum box to environmental disturbances (mechanical, electrical, etc.) and to generate indistinguishable (i.e., identical in terms of quantum state) single photons (same frequency, same polarization state, same spatial and temporal distribution).

[0006] Quantum boxes are usually of semiconducting nature obtained by molecular jet epitaxy, which consists in growing layers of semiconductors that differ in terms of gap energy and mesh parameters. Such differences create quantum boxes of nanometer dimensions that can confine charge carriers in three spatial dimensions.

[0007] An electron in the valence band of a quantum box can cross the conduction band to form an electron-hole pair (also called an exciton) in association with a hole left in the valence band. The formation of an exciton can occur by absorbing a photon with an energy above the stop band of the material forming the quantum box. The exciton state corresponding to the formation of an exciton is unstable, and the reconstruction of the neutral state, also called the ground state, occurs by the recombination of the electron and hole with the emission of a photon by a spontaneous radiative process characterized by a radiative lifetime on the order of nanoseconds and the emission wavelength (or frequency) corresponding to the stop band of the material forming the quantum box.

[0008] A quantum box can confine a biexciton, formed by two excitons bound by Coulomb interactions when they share the same ground state. The energy level of the biexciton state is higher than that occupied by a single exciton. The formation of a biexciton can occur through the sequential absorption of two photons. The relaxation of the biexciton also occurs through a spontaneous emission process, producing two photons that are sequentially emitted. Because the Coulomb interactions differ depending on whether one electron-hole pair or two electron-hole pairs are excited in the quantum box, the two photons are emitted at different wavelengths. Therefore, the photon emitted upon the transition of the system from the biexciton state to the exciton state is of a different wavelength than the photon emitted upon the transition of the system from the exciton state to the neutral state.

[0009] The polarization states of photons absorbed to form excitons and photons emitted upon electron-hole recombination obey optical selection rules governed by the Pauli exclusion principle and conservation of angular momentum. These selection rules depend on the spin state of the electron-hole pair and state that the "bright" exciton state that emits a photon upon their recombination is the one corresponding to the spin state of the opposite sign. When a quantum box has cylindrical symmetry, the system exhibits two energy-degenerate "bright" exciton states. Figure 1 illustrates the relaxation of a biexciton 103 formed in a semiconductor quantum box 100 with cylindrical symmetry (or "isotropy"), in which only "bright" exciton states are realized. Relaxation of a biexciton 103 confined in an isotropic quantum box 100 occurs in a system with three energy levels by sequentially emitting two photons. Thus, two pathways are possible: relaxation of the biexciton 103 to an exciton 1020 by emission of a right-handed circularly polarized photon, followed by relaxation of the exciton 1020 to the ground state 101 by emission of a left-handed circularly polarized photon; or Relaxation of the biexciton 103 to an exciton 1021 by emission of a left-handed circularly polarized photon, followed by relaxation of the exciton 1021 to the ground state 101 by emission of a right-handed circularly polarized photon.

[0010] Given that the two pathways are indistinguishable, this leads to the emission of a pair of polarization-entangled photons since the excitons 1020, 1021 are degenerate.

[0011] It should be noted that single photon generation can be achieved by exploiting the recombination of excitons 1020 or 1021 into the ground state 101, and that in this case going through the biexciton state 103 is irrelevant. The generation of a biexciton 103 is required for the generation of an entangled photon pair.

[0012] However, in reality, the semiconductor quantum boxes responsible for the lifting of exciton levels (and thus the loss of entanglement of the emitted photons) are anisotropic. This lifting of degeneracy is primarily induced by anisotropic effects in the quantum box's geometry, piezoelectric effects, mechanical constraints, etc. In anisotropic quantum boxes, the "bright" exciton states are separated by energy content (called "fine structure splitting") and are linear combinations of the fundamental states as defined in symmetric quantum boxes. The exciton states in anisotropic quantum boxes exhibit optical radiation that is linearly polarized along two specific axes "x" and "y" corresponding to the crystallographic directions (axis "z" is the growth axis). Figure 2 shows the recombination of a biexciton 203 in an anisotropic semiconductor quantum box 200. Relaxation of the biexciton 203 to the ground state 201 can then be carried out only according to a linear polarization state on one of the specific axes "x" or "y" of the quantum box, leaving the exciton 2020 or 2021 to relax according to the same polarization state as the biexciton 203. The lifting of the degeneracy of the exciton levels leads to a total or partial loss of entanglement between the two generated photons.

[0013] It is known practice to use anisotropic quantum boxes to generate single photons with improved performance levels by resonantly exciting a superposition of two "bright" exciton states and by exploiting the change in phase over time ΔΦ between the two exciton states polarized according to "x", "y", the rate (called the "transition rate" because it governs the transition from a state of linear polarization of the emitted photon to an orthogonal state, as will be explained in more detail with reference to FIG. 10) is proportional to the value of the fine structure splitting according to the following relation:

number

[0014] The use of fabricated anisotropic quantum boxes remains unsuitable for generating entangled photon pairs due to the lifting of the degeneracy of the exciton states. Therefore, there is a need to statically reduce the fine structure splitting (in particular, to offset the fine structure splitting) to reestablish the indistinguishability of the radiation emission pathways from the biexcitons to obtain entangled photon pairs.

[0015] It is a known approach (in this case, a mechanical approach) to reduce and / or control the fine structure splitting value within a quantum box by applying a perpendicular electric field along with one or more strain fields in two or three directions of the environment in which the quantum box is inserted. Such a solution was demonstrated in Reference [1]. The application of such strain fields induces thinning of the substrate on which the quantum box is produced in order to place the piezoelectric material as close as possible to the quantum box and to ensure that the applied mechanical tension is successfully transferred to the realized quantum box. The thinning required to implement such an approach presents a technological challenge for the fabrication of three-dimensional (3D) optical cavities incorporating quantum boxes. Furthermore, the proximity of the quantum box to the surface makes the quantum box sensitive to environmental disturbances that can induce a loss of indistinguishability of the generated single photons.

[0016] It is also known to apply an electric field within the two-dimensional space in which the quantum box is inserted to control the fine structure splitting value. Such a solution is implemented by inserting several (usually four) bonding pads (on a surface very close to the surface of the quantum box) (Reference [2]). It is known that the proximity of the quantum box to the surface on which such bonding pads are defined increases the charge noise and reduces the performance level of the emitted photons in terms of indistinguishability. [Prior art documents] [Non-patent literature]

[0017] [Non-Patent Document 1] Trotta, Rinaldo et al., “Highly Entangled Photons from Hybrid Piezoelectric Semiconductor Quantum Dot Devices.” Nano Letters, 14.6(2014):3439-3444. [Non-patent document 2] K Kowalik, O Krebs, A Lemaitre, S Laurent, P Senellart, P Voisin, JA Gaj, “Influence of an in-plane electric field on exciton fine structure inInAs-GaAs self-assembled quantum dots.”, Applied Physics Letters, 86(4), 041907 Summary of the Invention [Problem to be solved by the invention]

[0018] Therefore, there is a need for a source of single photons or entangled photon pairs based on semiconductor quantum boxes in which microstructure splitting can be statically or dynamically controlled without the drawbacks of the prior art. [Means for solving the problem]

[0019] General definition of the present invention To this end, the present invention provides a device for generating photons, the device comprising a quantum box inserted into a micropillar optical cavity having at least one optical mode, the quantum box having at least one ground state and two states with one elementary excitation, the cavity having a bottom surface and a top surface, the bottom surface having an electrical contact, and at least three electrical bonding pads electrically isolated from one another and arranged around the top surface of the cavity.

[0020] In one embodiment, the electrical bonding pads may be linked to the top surface of the cavity by semiconductor arms oriented radially relative to the cavity and having widths tangentially and at their ends closest to the cavity that are smaller than the width of the cavity.

[0021] In another embodiment, the electrical bonding pads may be linked to semiconductor arms oriented radially toward the cavity, and both ends of the semiconductor arms may be separated from the top surface of the cavity by submicron-wide voids or gaps filled with a dielectric.

[0022] Advantageously, the micropillar optical cavity may form a PIN-type diode, the quantum box being within the intrinsic region of the PIN-type diode.

[0023] In a variant, the electrical bonding pads are optically and electrically connected to the cavity. separated It can be carried by each pillar.

[0024] In one embodiment, the device may further include at least three adjustable voltage sources for applying respective variable potential differences between each of the electrical bonding pads and an electrical contact carried by the bottom surface of the optical cavity.

[0025] In another embodiment, the micropillar optical cavity may have at least a first pair and at least a second pair of modes, each pair being polarization degenerate, and the quantum box may also have one state with two elementary excitations.

[0026] Advantageously, the device may further comprise a second optical cavity coupled to the optical cavity into which the quantum box is inserted, the geometry of the first and second cavities and the strength of their coupling being selected such that the assembly consisting of the two coupled cavities exhibits a first pair of polarization-degenerate modes resonating with a transition between a state with two elementary excitations of the quantum box and two states with one elementary excitation; and a second pair of polarization-degenerate modes resonating with a transition between two states with one elementary excitation and the ground state of the quantum box, the modes of each pair having radiation patterns that overlap by more than 70%.

[0027] In particular, the second optical cavity may also be of the micropillar type, the two cavities being arranged side by side.

[0028] A method for generating entangled photon pairs by a photon generating device is proposed, which may include the steps of: applying a potential difference between each of the electrical bonding pads and an electrical contact carried by the bottom surface of the optical cavity, the potential difference being: Minimize the energy deviation between two states with one excitation of the quantum box, and selected to bring transitions between a state with two elementary excitations and a state with one elementary excitation into resonance with a first pair of modes of the cavity, and to bring transitions between a state with one elementary excitation and the ground state into resonance with a second pair of modes of the cavity; and -The process of filling a state of a quantum box with two elementary excitations by inputting energy.

[0029] A method for generating single photons by a photon generating device is also proposed, which may include the steps of: applying a potential difference between each of the electrical bonding pads and an electrical contact carried by the bottom surface of the optical cavity; and - illuminating the quantum boxes of the device with light pulses exhibiting a spectrum and direction of polarization corresponding to the modes of the optical cavity, the potential difference being chosen to modify the energies of two eigenstates with one elementary excitation in the following way: The state is resonant with a wavelength in the spectrum of the light pulse; and The probability of emission by the quantum box of a single photon with linear polarization orthogonal to the polarization of the illumination is maximized.

[0030] In one embodiment, the potential difference may also be selected to orient a particular axis of the quantum box according to an intermediate direction between the directions of two particular axes of polarization of the optical cavity.

[0031] In another embodiment, the potential difference may be variable over time and: - during at least a portion of a first period, the quantum box is illuminated and assumes first values ​​selected for the energies of two eigenstates having one elementary excitation to have a non-minimal deviation, the first period having a duration selected to induce a phase change between the two states of 45° and 135° (preferably 80° and 100°); and During a second period immediately following the first period, it takes a value selected to minimize the deviation between the energies of two eigenstates with one elementary excitation.

[0032] Other features and advantages of the present invention will become apparent from the following detailed description and accompanying drawings. [Brief explanation of the drawings]

[0033] [Figure 1] The relaxation of a biexciton in the isotropic quantum box described above is shown. [Figure 2] The relaxation of biexcitons in the anisotropic quantum box described above is shown. [Figure 3A] 1 illustrates a device for generating single photons and entangled photon pairs according to one embodiment of the present invention. [Figure 3B] 1 illustrates a device for generating single photons and entangled photon pairs according to one embodiment of the present invention. [Figure 4] 1 illustrates the generation of an electric field by three components in space according to one embodiment of the present invention. [Figure 5] 1A and 1B illustrate a portion of a top view of a device for generating single photons and entangled photon pairs according to an embodiment of the present invention. [Figure 6] 1A and 1B illustrate a portion of a top view of a device for generating single photons and entangled photon pairs according to an embodiment of the present invention. [Figure 7] 1A and 1B illustrate a portion of a top view of a device for generating single photons and entangled photon pairs according to an embodiment of the present invention. [Figure 8] 1 illustrates a micropillar optical cavity incorporating a quantum box according to one embodiment of the present invention; [Figure 9]10 illustrates a portion of a top view of a device for generating single photons and entangled photon pairs implementing two optically coupled optical cavities according to another embodiment of the present invention. [Figure 10] The time evolution of the excited state of a quantum box trapped in a micropillar optical cavity is shown, where the quantum box exhibits non-zero exchange anisotropy and the optical cavity exhibits birefringence. [Figure 11] 1 illustrates the use of a device for generating single photons according to one embodiment of the present invention. [Figure 12] 1 shows a method for calibrating anisotropic quantum boxes. [Figure 13A] 1 illustrates a method for generating a single photon according to one embodiment of the present invention. [Figure 13B] 1 illustrates a method for generating a single photon according to one embodiment of the present invention. [Figure 14] 1 illustrates a method for generating entangled photon pairs according to one embodiment of the present invention. [Figure 15] 10 illustrates the measured microstructural division of anisotropic quantum boxes exposed to an electric field generated in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] 3A and 3B show top and cross-sectional views, respectively, of a device 300 for generating single photons and entangled photon pairs according to one embodiment of the present invention. The device includes a quantum box 301 inserted into a micropillar optical cavity 302. The optical cavity 302 has a cylindrical shape and is connected to three pillars 303 by semiconductor arms 308a, 308b, and 308c.

[0035] The quantum box 301 may be of semiconductor nature obtained by a molecular jet growth method, which consists in growing semiconductor layers that differ in terms of gap energy and mesh parameters. The layers forming the quantum box 301 may be layers of indium arsenide (InAs). The layers forming the substrate and capsule may be layers of gallium arsenide (GaAs). The quantum box 301 may have a truncated conical shape with a height of a few nanometers in the growth axis direction and a base diameter of about 20 nanometers. These characteristics give the quantum box 301 properties that are close to those of an atom in terms of charge carrier confinement and energy level quantization.

[0036] The quantum box 301 can be in a neutral state (also called the ground state), or in an excited state corresponding to the formation of an electron-hole pair (also called an exciton), or in a state with one elementary excitation. The formation of an exciton in the presence of charge carriers already trapped in the quantum box 301 corresponds to a trion, which can be quantified as positive or negative depending on the sign of the already trapped charge carrier. A biexciton (also called a state with two elementary excitations) is formed in the quantum box 301 when two excitons are simultaneously trapped.

[0037] Charge carriers, electrons and holes, are fermions characterized by half-integer spin (±½ for electrons and ±¾ for heavy holes). The exciton states of the symmetric quantum box 301 are bright if the electron-hole recombination occurs according to a spontaneous radiative process, the essence of which is the emission of a photon. The bright exciton state (also called the fundamental exciton state) corresponds to a total angular moment equal to ±1 and relaxes by emitting a left- or right-handed circularly polarized photon, depending on the sign of the spin sum.

[0038] In fact, quantum box 301 is anisotropic (meaning the degeneracy of the exciton levels in quantum box 301 is lifted). The "bright" exciton states (also called eigenstates with one elementary exciton) in anisotropic quantum box 301 are linear combinations of the elementary exciton states defined in symmetric quantum box 301 and are separated by a non-zero amount of energy called the fine structure splitting.

[0039] The micropillars forming the optical cavity 302 are cylindrical in shape and are two stacks of layers 3020, 3022 surrounding a central region 3021. Such layers are usually semiconducting in nature and can be P- or N-doped according to a given configuration. For example, the top of the cavity 3022 can have P-type (or N-type) doping, while the bottom 3020 can have the opposite doping, so that the cavity 3020 forms a PN diode from an electrical standpoint. Each of the two stacks of layers 3020, 3022 forms a Bragg mirror and is obtained by alternating two layers of two different materials in terms of refractive index. The thickness and refractive index of each of the two layers forming each of the two stacks 3020, 3022 can be selected so that their product is equal to λ / 4, where λ is the operating frequency. The number of layer pairs forming the bottom stack 3020 in contact with the substrate 305 can be different from the number of layer pairs forming the top stack 3022. Such a difference allows, for example, to prioritize the upward emission of photons generated by the quantum box 301 in order to increase the brightness of the photon source 300. Each of the two Bragg mirrors 3020, 3022 allows incident light to be refracted along the stack axis over a wide wavelength range centered around the operating frequency λ. A central region 3021, located between the two stacks 3020, 3022, can have a thickness equal to λ and is configured to contain the quantum box 301. The presence of the central region 3021 allows the refractive index of the micropillars to be modified so that the two Bragg mirrors 3020, 3022 are transparent at the operating frequency. The radius of the micropillars is on the order of a few micrometers and is selected to allow at least one optical mode, called the fundamental mode, to propagate through the micropillar with minimal energy. Advantageously, the radius of the micropillars can be selected to allow the propagation of several optical modes. The micropillars can have rotational symmetry, which implies two degenerate polarization states for each propagation mode. Alternatively, the micropillars are birefringent media in which light propagates anisotropically. This property of birefringence removes the degeneracy of polarization of the light propagation mode defined by the minor and major axes (denoted H and V, respectively) of the micropillars.

[0040] Advantageously, the optical cavity 302 has at least one first pair and at least one second pair of optical modes, each of which mode pairs is polarization degenerate. The optical cavity 302 having two pairs of polarization degenerate modes is particularly used in the generation of entangled photon pairs, for example by matching the energy of the first pair of modes with the wavelength of relaxation from a biexciton (a state with two elementary excitations) to an exciton (a state with one elementary excitation) and by matching the energy of the second pair of modes with the wavelength of relaxation from an exciton to a neutral state.

[0041] The pillars 303 may have a height equal to, greater than, or less than the pillars of the optical cavity 302. The optical cavity 302 is optically isolated from the pillars 303. Separate The assembly formed by the three pillars 303 can be centered according to a radial spacing sufficient to allow for electrical contacts 305, which will be discussed later. Furthermore, the pillars 303, which correlate with the electrical contacts 305, must be electrically isolated from each other except at their bottom surfaces. The pillars 303 can be formed by the same layer stack as the micropillars that form the optical cavity 302. Such a structure offers the advantage of being compatible with numerous cleanroom manufacturing methods known to those skilled in the art. The fabrication process for such a structure can include thin layer deposition methods, lithography methods, to generate the layer stacks, followed by dry etching methods that allow for the definition of the optical cavity 302 and the three pillars 303.

[0042] The top surface of each pillar 303 (defined as the surface opposite the surface that contacts the substrate 305) includes an electrical bond pad 304a, 304b, 304c, the dimensions of which are selected so that the electrical bond pad 304a, 304b, 304c is completely carried by the top surface of the corresponding pillar 303. The three electrical bond pads 304a, 304b, 304c may have the same electrical properties and the same geometric shape, which may be triangular, rectangular, or other. The lateral dimensions of such electrical bond pads 304a, 304b, 304c, defined in a plane perpendicular to the stacking axis (growth axis), are preferably at least a factor of 10 greater than the thickness of such pads 304a, 304b, 304c, defined according to the stacking axis. By way of example, the thicknesses of the electrical bonding pads 304a, 304b, 304c are selected to be less than 50 nanometers and their lateral dimensions are selected to be greater than 50 micrometers, allowing for easy connection thereof to different voltage sources 306a, 306b, 306c by a wiring method also known as "wire bonding." The electrical bonding pads 304a, 304b, 304c are arranged symmetrically around the optical cavity 302 (corresponding to the same radial spacing between each of the electrical bonding pads 304a, 304b, 304c and the optical cavity 302 and the same angular spacing between two adjacent electrical bonding pads).

[0043] The bottom surface of the pillar 303 is connected to another electrical contact 305, which is therefore carried by the bottom surface of the optical cavity 302. The electrical contact 305 is fabricated on a surface of semiconducting nature, and its conductivity is increased through P or N doping. The electrical contact 305 may have a disk shape, a rectangular shape, or another 2D geometric shape that allows it to completely cover the bottom surface of the assembly formed by the optical cavity 302 and the pillar 303. Such a configuration makes it possible to apply a potential difference between each of the electrical bonding pads 304a, 304b, and 304c located around the top surface of the optical cavity 302 and the electrical contact 305 carried by the bottom surface of the optical cavity 302 by using three independent voltage sources 306a, 306b, and 306c that deliver respective voltages Ta, Tb, and Tc. The voltages delivered by each of the voltage sources 306a, 306b, and 306c may be constant or may vary over time.

[0044] Arms 308a, 308b, 308c of semiconductor nature are used to connect each of the electrical bonding pads 304a, 304b, 304c to the top surface of the optical cavity 302. The material forming the semiconductor arms is doped to increase their electrical conductivity, but this conductivity must not become metallic-type (degenerate doping). The semiconductor arms 308a, 308b, 308c may have the same height as the optical cavity 302, their length is long enough to connect the outer surface of the optical cavity 302 to the inner surface of the pillar 303 (on which the electrical bonding pads 304a, 304b, 304c are located), and their width is small enough so that two adjacent semiconductor arms 308a, 308b, 308c are electrically isolated from their contact with the outer surface of the optical cavity 302. Each of the semiconductor arms 308a, 308b, 308c extends radially towards the corresponding pillar 303 such that an angle of 120 degrees is formed between two adjacent semiconductor arms 308a, 308b, 308c. The use of dielectric or semiconductor arms makes it possible to amplify the horizontal component of the applied electric field by concentrating the field lines.

[0045] Photons generated by and exciting the quantum box 301 according to one of the optical modes exit or enter the optical cavity 302, respectively, according to the same or two different radiation patterns 307, each characterized by a given aperture angle.

[0046] The potential difference between each of the electrical bonding pads 304a, 304b, and 304c and the electrical contact 305 borne by the bottom surface of the optical cavity 302 generates an elementary electric field and controls its characteristics in terms of intensity and orientation. The resultant electric field prevailing in the central region 3021 and to which the quantum box 301 is subjected is the vector sum of three elementary electric fields derived from the potential difference between each of the three electrical bonding pads 304a, 304b, and 304c arranged around the top surface of the optical cavity 302 and the electrical contact 305 borne by the bottom surface of the optical cavity 302. This resultant electric field has three components: a vertical component defined in the same direction as the growth axis (layer axis) of the optical cavity 302 and two horizontal components defined in the plane of the layers forming the two Bragg mirrors 3020 and 3022. The characteristics of this resultant electric field in terms of orientation and intensity can be adjusted in a deterministic manner by acting on one or more elementary electric fields via the associated potential difference.

[0047] 4 illustrates the generation of an electric field having three components defined in an orthonormal trihedron by introducing four electrical contacts according to the configuration described above, where two voltage sources 306a, 306b are represented and the base and resultant electric fields are represented by dotted and continuous lines, respectively. The various components of the resultant electric field are: -adjusting the fine structure division value of the quantum box 301; - adjusting the emission wavelength associated with the relaxation of that "bright" exciton state; and It makes it possible to define the orientation of a particular axis of the quantum box 301 according to which it can emit photons or be excited by photons.

[0048] To adjust these three parameters, it is necessary to have at least three degrees of freedom, and therefore at least three independent voltage sources are linked to three non-collinear electrical bonding pads.

[0049] From an experimental point of view, the intrinsic parameters of the quantum box 301 (such as the fine structure division) are not precisely known in advance due to imperfections in the fabrication method. The parameters of interest of the quantum box 301 (such as the fine structure division, emission wavelength, and orientation of specific axes) are usually measured after fabrication of the device for generating single photons and entangled photon pairs 300.

[0050] FIG. 5 shows a top view of a portion of a device for generating single photons and entangled photon pairs 300 according to another embodiment of the present invention. In this embodiment, the aforementioned semiconductor arms 308a, 308b, and 308c are not in direct contact with the optical cavity 302. Separation of each of the semiconductor arms 308a, 308b, and 308c from the optical cavity 302 is ensured by a dielectric layer 309. The dielectric layer 309 is configured to ensure electrical isolation between the optical cavity 302 and the semiconductor arms 308a, 308b, and 308c, and its thickness is on the order of tens of nanometers to several micrometers. The dielectric material forming the dielectric layer 309 can be air. Electrical isolation of the optical cavity 302 from its surrounding environment makes it possible to avoid any strong currents passing through the optical cavity 302 and disturbing the operation of the quantum box 301.

[0051] 6 illustrates another embodiment of the present invention in which no semiconductor arms are used to connect the optical cavity 302 to the pillars 303 realized. In such an embodiment, the pillars 303 are arranged symmetrically around the optical cavity 302, and the radial spacing between the optical cavity 302 and each of the pillars 303 is selected to be, for example, less than 10 micrometers. Such a radial spacing makes it possible to expose the quantum box 301 to an electric field strong enough to affect the characteristic parameters of the quantum box 301, including the fine structure splitting value. The separation between the optical cavity 302 and the pillars 303 can be ensured by a dielectric material or an air gap.

[0052] 7 illustrates another embodiment of the present invention in which the electrical bonding pads 304a, 304b, and 304c are carried by a cylindrical hollow structure 3030 that surrounds the optical cavity 302. The hollow structure 3030 may be formed of a dielectric material that provides electrical isolation between the electrical bonding pads 304a, 304b, and 304c. The hollow structure 3030 may have the same height as the optical cavity 302, with its top surface selected to be wide enough to contain the electrical bonding pads 304a, 304b, and 304c. Furthermore, semiconductor arms 308a, 308b, and 308c associated with the dielectric layer 309 described in the embodiment of FIG. 5 are used to connect the electrical bonding pads 304a, 304b, and 304c to the optical cavity 302. Alternatively, the connection between the electrical bonding pads 304a, 304b and the optical cavity 302 can be ensured by bringing the electrical bonding pads 304a, 304, 304c sufficiently close to the optical cavity 302 as described in FIG. 6 or by using the semiconductor arms 308a, 308b, 308c as described in FIGS. 3A and 3B.

[0053] FIG. 8 illustrates the structure of a micropillar optical cavity 302 according to one embodiment of the present invention. In such an embodiment, the two top and bottom Bragg mirrors 3022 and 3020 forming the micropillar are doped with P and N, respectively. The central region 3021 containing the quantum box 301 remains intrinsic so that the micropillar forms a PIN-type diode from an electrical standpoint. The concentration of the impurity used to create the doping (P or N) can be uniform throughout the entire length of each of the two Bragg mirrors 3020 and 3022. Alternatively, the impurity concentration can vary in a tapered manner so as to be minimal in the region in contact with the central region 3021. The variation in the impurity concentration can be linear, logarithmic, or other. Advantageously, different voltages applied to an optical cavity 302 doped according to one of the configurations described above correspond to opposite polarities of the PIN junction formed by the optical cavity 302. Such opposite polarities ensure that no significant current passes through the quantum box 301 and disturbs its emission of photons. Alternatively, optical cavity 302 is forward biased, which would likely generate a large current through quantum box 301 and disrupt its operation.

[0054] According to some embodiments of the present invention, each of the electrical bonding pads 304a, 304b, 304c surrounding the top surface of the optical cavity 302 is connected to one terminal of an adjustable voltage source 306a, 306b, 306c. Each of the adjustable voltage sources 306a, 306b, 306c is configured to apply a potential difference between its connected electrical bonding pad 304a, 304b, 304c and an electrical contact 305 carried by the bottom surface of the optical cavity 302 that is common to all implemented adjustable voltage sources 306a, 306b, 306c. The voltage generated by each of the adjustable voltage sources 306a, 306b, 306c, measured between the corresponding electrical bonding pad 304a, 304b, 304c and the electrical contact 305, can be positive, zero, or negative.

[0055] FIG. 9 shows a top view of a device for generating single photons and entangled photon pairs 300, including two optical cavities 302, 310, according to another embodiment of the present invention. One of the two optical cavities 302 includes a quantum box 301 and may be configured according to one of the embodiments described above. The other optical cavity 310 does not include a quantum box and may have different optical geometric parameters (size, shape) than the first optical cavity 302. The two realized optical cavities 302, 310 are configured to be optically coupled according to a given coupling strength. The assembly formed by the two optical cavities 302, 310 has two pairs of polarization-degenerate optical modes. The first pair of polarization-degenerate optical modes is configured to resonate with a transition between a state with two elementary excitations (biexciton) and two states with one elementary excitation (exciton). The second pair of polarization-degenerate optical modes is configured to resonate with the transition between two states with one elementary excitation (exciton) of the quantum box 301 and the ground state, also called the neutral state. The modes of each pair of optical modes have radiation patterns 307 that overlap by more than 70%. The coupling between the two optical cavities 302, 310 can also be configured according to the embodiment described in patent application [3].

[0056] According to some embodiments of the present invention, the number of electrical bonding pads disposed around the periphery of the top surface of optical cavity 302 is greater than three. This number of electrical bonding pads may be even or odd, and may be less than 20. The electrical bonding pads may be disposed in an asymmetric manner around optical cavity 302 such that at least three electrical bonding pads (and possibly their associated semiconductor arms) are not oriented in a pairwise parallel direction. This means that the angular and radial spacing between adjacent electrical bonding pads, as measured from optical cavity 302, may not be constant for all electrical bonding pads implemented. Furthermore, the electrical bonding pads may be located at different distances from electrical contact 305; in other words, the electrical bonding pads may be located in different planes perpendicular to the stacking axis.

[0057] According to some embodiments of the present invention, a device for generating single photons and entangled photon pairs 300 may include a cooling unit (e.g., by the Peltier effect or by nitrogen or helium cryogenics) configured to control and keep constant the operating temperature of the quantum box 301. Such an operating temperature may be between 4 Kelvin and 100 Kelvin. Such a low operating temperature limits the interaction of the quantum box 301 with its immediate environment and allows the coherence of the excited and ground states to be maintained over a time interval long enough to manipulate the state of the quantum box 301.

[0058] FIG. 10 shows the time evolution 400 of a "bright" exciton state in a quantum box 301 inserted into a micropillar-based optical cavity 302, where the quantum box 301 and the optical cavity 302 are characterized by nonzero anisotropy. The anisotropy of the optical cavity 302 refers to the lifting of the degeneracy between the modes of the optical cavity 302 with low-energy modes and high-energy modes that are aligned with a specific axis (H, V) of the micropillar's polarization. The anisotropy of the quantum box 301 refers to the lifting of the degeneracy between two excited states separated by a nonzero amount of energy called the fine structure splitting. Each of the two excited states (also called eigenstates) of the quantum box 301 is associated with a given linear polarization state X or Y that can emit a photon or be excited by a photon. In general, the modes (H, V) of the optical cavity 302 will not coincide with the direction of polarization (x, y) of the quantum box 301, and an angle denoted θ can be defined to quantify this mismatch. Resonant excitation of the exciton level of the quantum box 301 by photons linearly polarized according to one of the modes of the optical cavity 302 (H or V) leads to the excitation of two eigenstates, with a weighting that depends on the value of the angle θ. The resulting excited state 4021 or 4020 is not stable, and its phase changes over time with a transition rate proportional to the fine structure splitting. This behavior can be exploited to collect only single photons generated by the quantum box 301, for example, by exciting with photons linearly polarized according to the axis H of the optical cavity 302 and collecting only photons generated according to a linear polarization state that is perpendicular to the angle used during excitation (in this case, the axis V of the optical cavity 302). In such a configuration, increasing the fine structure splitting allows photons to be emitted in the direction of the detection polarization before being spontaneously emitted in the direction of the excitation polarization. In a second step, the fine structure splitting should be reduced to limit the probability of two photons being emitted instead of one within the same excitation region.

[0059] FIG. 11 illustrates a device for generating single photons 300 according to one embodiment of the present invention. Such an embodiment implements an anisotropic quantum box 301 incorporated within a micropillar-type birefringent optical cavity 302. The quantum box 301 is subjected to an electric field of adjustable strength and orientation via electrical bonding pads 304a, 304b, 304c, and 305 arranged according to one of the embodiments described above. Pad 304c and associated voltage source 306c are hidden in FIG. 11. The device for generating single photons 300 includes an excitation laser source 501 configured to generate an optical pulse intended to optically excite the quantum box 301 to convert its state from a neutral state to an excited state. The state of polarization of the optical pulse is linear along one of the specific axes of the optical cavity 302, and the associated wavelength is selected so that the energy carried by the photon is large enough to excite the quantum box 301. The duration of the optical pulse and its repetition frequency determine the rate of the number of photons per second generated by the device for generating single photons 300. The device for generating single photons 300 further includes a polarization splitter cube 502 and a converging lens 503 configured to split the photons generated in the optical cavity 302 according to a linear polarization state in a direction perpendicular to the direction of the excitation photons and to improve the optical coupling between the polarization splitter cube 502 and the optical cavity 302, respectively. The specific axes (H, V) of the optical cavity 302 are configured to be non-collinear with the specific axes (x, y) of the quantum box 301 according to an angle θ (ideally equal to 45 degrees).

[0060] FIG. 12 illustrates a method 600 for calibrating an anisotropic quantum box 301 subjected to an electric field as shown in FIG. 4. The three spatial components of the resultant electric field are independently adjustable by at least three adjustable voltage sources. The method is iterative. For each iteration, the method consists in modifying the intensity of at least one of the three components of the resultant electric field (step 601) and measuring, in a second step, a characteristic parameter or parameters of interest of the quantum box 301 associated with the applied resultant electric field (step 602). The method can be stopped when the desired value of the characteristic parameter of interest is obtained, and the values ​​of the voltages applied to the various electrical bonding pads can be restored. The first iteration of the method can correspond to a resultant electric field of zero intensity.

[0061] A calibration method can be used to identify the operating point of the device for generating entangled photon pairs 300. Characteristic parameters of interest for such a device include the fine structure splitting, which must be significantly reduced (ideally set to zero), and the emission wavelengths corresponding to the relaxation of the biexciton and the relaxation of the exciton.

[0062] The present calibration method can further be used to find the optimum operating point of the device for generating single photons 300 operating in static mode, according to which the fine structure splitting characterizing the anisotropic quantum box 301 to be realized is statically adjusted before the generation of any excitons. The fine structure splitting must allow a sufficient speed of transition so that the excited state can relax according to a linear polarization state lying at right angles to the angle of the excitation photon at the end of the transition time, which is less than the duration of the excitation light pulse. The fine structure splitting is further optimized as follows: - greater than the lower limit to which the excited state can relax according to a linear polarization state perpendicular to the angle of the excitation photon at the end of a transition time that is at least a factor of 3 (preferably at least a factor of 5) shorter than the spontaneous emission lifetime of the exciton state; and - so that the quantum box 301 is smaller than the upper limit at which it can emit more than one photon during the duration of the excitation light pulse.

[0063] FIG. 13A is a flow chart of a method 700a for generating single photons using a device for generating photons 300 according to one embodiment of the present invention. In such an embodiment, the microstructure division characterizing the anisotropic quantum box 301 is statically adjusted. The first step 701a of the method consists in applying a potential difference between each of the electrical bonding pads 304a, 304b, and 304c and the electrical contact 305 carried by the bottom surface of the optical cavity 302, so as to generate a resultant electric field to which the quantum box 301 is subjected. The components of this resultant electric field are adjusted to optimize the value of the microstructure division, as described above. In addition to matching the wavelength of the single photons generated by the quantum box 301, the application of the resultant electric field in three directions in space allows defining the orientation of a specific axis of the quantum box 301 so that it is not collinear with the specific axis of the optical cavity 302 into which the quantum box 301 is inserted. Advantageously, the specific axis of the quantum box 301 is oriented at 45 degrees relative to the specific axis of the optical cavity 302.

[0064] The second step 702a of the method consists in illuminating the quantum box 301 with a series of light pulses whose wavelength spectrum and polarization state correspond to one of the modes of the optical cavity 302 in which the quantum box 301 is inserted. The duration of the light pulses is selected so that they are long enough to excite the quantum box 301 and allow it to emit at least one photon according to the spontaneous emission process.

[0065] 13B is a flow diagram of a method for generating a single photon 700b by a device for generating photons 300 according to another embodiment of the present invention. In such an embodiment, the fine structure splitting characterizing the anisotropic quantum boxes 301 is dynamically controlled. Such dynamic control is achieved through modulation of the electric field to which the quantum boxes 301 are exposed such that the fine structure splitting value can be modified within a time interval shorter than the spontaneous emission time characterizing the quantum boxes 301.

[0066] The first step 701b of the method consists in illuminating the quantum box 301 with a series of light pulses whose wavelength spectrum and polarization state correspond to the modes of the optical cavity 302 populating the quantum box 301. The duration of the light pulses is selected so that they are long enough to excite the quantum box and allow it to emit at least one photon according to the spontaneous emission process.

[0067] The second step 702b of the method consists in applying a potential difference between each of the electrical bonding pads 304a, 304b, and 304c and the electrical contact 305 carried by the bottom surface of the optical cavity 302, so as to generate a resultant electric field to which the quantum box 301 is subjected. The fine structure division value increases depending on the strength of the resultant electric field, which must remain below the threshold corresponding to damage to the quantum box 301 (i.e., the threshold at which the electrical and optical characteristics of the quantum box 301 can no longer be reconstructed, as determined during calibration). The strength of the resultant electric field is selected to be sufficiently high so that the change over time of the phase of the generated excited state lies between 45° and 135° (preferably between 80° and 100°). This change over time of the phase must also occur within a time shorter (by a factor of at least 3, preferably at least 5) than the spontaneous emission lifetime of the exciton state.

[0068] The third step 703b of the method can be triggered immediately after the required change in the phase of the initial exciton state over time is completed. Such a step of the method consists in modifying the strength of the resulting electric field so as to minimize the energy deviation between two eigenstates with one elementary excitation.

[0069] 14 illustrates a method for generating entangled photon pairs 800 by a device for generating photons 300 according to an embodiment of the present invention. Such a device for generating photons 300 implements an anisotropic quantum box 301 incorporated in an optical cavity 302 having at least one first pair of optical modes and at least one second pair of optical modes, each of which is polarization-degenerate. The entanglement of the generated photons is obtained thanks to static control of the fine structure division that characterizes the implemented anisotropic quantum box 301. A first step 801 of the method consists in applying a potential difference between each of the electrical bonding pads 304a, 304b, 304c and an electrical contact 305 carried by the bottom surface of the optical cavity 302. Such a potential difference is optimized to reduce the energy deviation between two states with one elementary excitation defined by the fine structure division of the quantum box 301. In particular, step 801 consists in counteracting the fine structure splitting that makes quantum box 301 a system with three energy levels involved in the relaxation of a biexciton. The potential difference is also set as follows: - such that the transition between a state with two elementary excitations and a state with one elementary excitation is in resonance with the first pair of modes of the optical cavity 302; and - the transition between the state with one elementary excitation and the ground state is in resonance with the second pair of modes of the optical cavity 302.

[0070] The second step 802 of the method consists in filling a state with two elementary excitations of the quantum box 301 by inputting energy. The creation of such a biexcited state XX can be produced by exciting the quantum box 301 with two successive photons of the following suitable energy: ω with energy sufficient to excite the transition of the first exciton X and ω for generating the second exciton and thus obtaining the biexcited state XX. X energy ω different from XX the second photon, or alternatively with energy (ω XX +ω X ) / 2 two photons.

[0071] FIG. 15 shows experimental measurements of the fine structure splitting characterizing an anisotropic quantum box 301 incorporated within a micropillar-based optical cavity 302, the top surface of which is surrounded by two electrical bonding pads 304a, 304b in accordance with an embodiment of the present invention. The experimental measurements were performed for two ranges of voltages applied to the electrical bonding pads 304a, 304b and demonstrate the technical advantage of embodiments of the present invention in terms of deterministic control of the fine structure splitting. These measurements were obtained for the following configuration: the quantum box 301 is excited by a laser with an energy greater than the optical transition corresponding to the exciton. This non-resonant excitation allows the exciton states 2020, 2021 to fill with energy input, which then emits photons at two different wavelengths that are collected. The wavelength difference between the two photons allows the fine structure splitting to be extracted. Figure 15 shows the results of such a measurement, where the color scale reflects the magnitude of the fine structure division as a function of the two voltages applied to contacts 304a, 304b. The figure shows that the fine structure division changes magnitude as a function of the applied voltage, and that the fine structure division changes sign by canceling out for the entire range of combinations of Voltage 1 and Voltage 2.

[0072] According to one embodiment of the present invention, the formation of excitons in the initially neutral quantum box 301 is achieved by non-resonant optical pumping, which consists in exciting the quantum box 301 with photons of energy above the stop band of the material forming the quantum box 301.

[0073] According to another embodiment of the present invention, the geometric shape of the micropillars forming the optical cavity 302 defined in a plane perpendicular to the growth axis can be elliptical, polygonal, or other.

[0074] The invention is not limited to the embodiments described above as non-limiting examples, but encompasses all variant embodiments that become conceivable by a person skilled in the art.

[0075] References [1]Trotta,Rinaldo et al.”Highly Entangled Photons from Hybrid Piezoelectric Semiconductor Quantum Dot Devices.”Nano Letters 14.6(2014):3439-3444. [2]K Kowalik,O Krebs,A Lemaitre,S Laurent,P Senellart,P Voisin,JA Gaj”Influence of an in-plane electric field on exciton fine structure inInAs-GaAs self-assembled quantum dots.”Applied Physics Letters 86(4),041907 [3]WO / 2011 / 089336”SOURCE OF POLARIZATION-ENTANGLED PHOTON PAIRS AND METHOD FOR MANUFACTURING SAME”.

Claims

1. 1. A device for generating photons, comprising a quantum box (301) inserted into a micropillar-type first optical cavity (302) having at least one optical mode, The quantum box (301) has two states having at least one ground state and one elementary excitation, the first optical cavity (302) has a bottom surface and a top surface, the bottom surface having an electrical contact (305), The device includes at least three electrical bonding pads (304a, 304b, 304c) electrically isolated from one another and arranged around a periphery of a top surface of the first optical cavity (302), The device also includes at least three adjustable voltage sources (306a, 306b, 306c) for applying a variable potential difference between each of the electrical bonding pads (304a, 304b, 304c) and the electrical contact (305) carried by the bottom surface of the first optical cavity (302).

2. 2. The device of claim 1, wherein the electrical bonding pads (304a, 304b, 304c) are linked to the top surface of the first optical cavity (302) by semiconductor arms (308a, 308b, 308c) oriented radially relative to the first optical cavity (302) and having a width smaller than the first optical cavity (302) in the tangential direction and at their ends closest to the first optical cavity.

3. 2. The device of claim 1, wherein the electrical bonding pads (304a, 304b, 304c) are linked to semiconductor arms oriented radially toward the first optical cavity (302).

4. A device described in any one of claims 1 to 3, wherein the micropillar-type first optical cavity (302) forms a PIN-type diode, and the quantum box (301) is located within the intrinsic region (3021) of the diode.

5. The device of any one of claims 1 to 4, wherein the electrical bonding pads (304a, 304b, 304c) are carried by respective pillars (303) that are optically and electrically isolated from the first optical cavity (302).

6. The device according to any one of claims 1 to 5, wherein the micropillar-type first optical cavity (302) has at least one first pair and at least one second pair of modes, each of the pair of modes being polarization degenerate, and the quantum box (301) also has one state with two elementary excitations.

7. 7. The device of claim 6, further comprising a second optical cavity (310) coupled to the first optical cavity (302) into which the quantum box (301) is inserted, the geometry of the first optical cavity and the second optical cavity (302, 310) and the strength of their coupling are selected such that the device consisting of the two coupled optical cavities has a first pair of polarization-degenerate modes that resonate with the transitions between the state with two elementary excitations of the quantum box (301) and the two states with one elementary excitation; and a second pair of polarization-degenerate modes that resonate with the transitions between the two states with one elementary excitation of the quantum box (301) and the ground state, wherein the modes of each pair of the first pair and the second pair have radiation patterns that overlap by 70% or more.

8. The device of claim 7 , wherein the second optical cavity (310) is also a micropillar type, and the two optical cavities (302, 310) are arranged side by side.

9. 10. A method of generating entangled photon pairs with the device of claim 7, the method comprising: applying a potential difference between each of the electrical bonding pads (304a, 304b, 304c) and the electrical contact (305) carried by the bottom surface of the first optical cavity (302), said potential difference being: so as to minimize the energy difference between the two states with one excitation of the quantum box (301); and selected to cause the transition between the state with two elementary excitations and the state with one elementary excitation to be in resonance with the first pair of modes of the first optical cavity (302), and the transition between the state with one elementary excitation and the ground state to be in resonance with the second pair of modes of the first optical cavity (302); and - filling said state with two elementary excitations of said quantum box (301) by inputting energy.

10. 10. A method for generating single photons by a device according to any one of claims 1 to 6, said method comprising: applying a potential difference between each of the electrical bonding pads (304a, 304b, 304c) and the electrical contact (305) carried by the bottom surface of the optical cavity (302); and - illuminating the quantum boxes (301) of the device with light pulses exhibiting a spectrum and direction of polarization corresponding to the modes of the optical cavity (302), The potential difference is chosen to modify the energies of two eigenstates with one elementary excitation in the following manner: the state is resonant with a wavelength in the spectrum of the light pulse; and A method in which the probability of emission by said quantum box (301) of a single photon exhibiting a linear polarization orthogonal to said polarization is maximized.

11. 11. The method of claim 10, wherein the potential difference is also selected to orient a particular axis of the quantum box (301) according to an intermediate direction between directions of two particular axes of polarization of the optical cavity (302).

12. The potential difference is variable with time: - during a first period, during at least a portion of which the quantum box (301) is illuminated, it takes on first values ​​selected for the energies of the two eigenstates having one elementary excitation to exhibit a non-minimal deviation, the first period having a duration selected to induce a phase change between 45° and 135° or between 80° and 100° between the two states; 12. The method according to claim 10, wherein during a second period immediately following the first period, the energy of two eigenstates having one elementary excitation takes a value selected to minimize the deviation between the energies of two eigenstates.

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